Multi-layer microporous film for batteries having shut-off function

ABSTRACT

The invention relates to a biaxially oriented, multilayer microporous foil including one layer made from propylene homopolymer, a propylene block copolymer, a polyethylene and β-nucleating agent, and at least one further porous layer.

The present invention relates to a multilayer microporous foil and usethereof as a separator.

Modern devices rely on an energy source, such as batteries orrechargeable batteries, that enable the devices to be used in anylocation. Batteries have the disadvantage that they must be disposed of.Therefore, the use of rechargeable batteries (secondary batteries) thatcan be recharged repeatedly with the aid of chargers plugged into themains is becoming more and more widespread. Nickel-cadmium rechargeablebatteries (NiCd rechargeable batteries), for example, have a servicelife of about 1000 recharging cycles if they are used correctly.

Batteries and rechargeable batteries always consist of two electrodeswhich are immersed in an electrolyte solution, and a separator, whichseparates the anode and the cathode from one another. The various typesof rechargeable battery differ in the electrode material, theelectrolyte, and the separator used. A battery separator has the task ofkeeping apart the cathode and the anode in batteries, or the negativeand the positive electrode in rechargeable batteries. The separator mustbe a barrier that insulates the two electrodes from each other, toprevent internal short circuits. Yet at the same time the separator mustbe permeable for ions so that the electrochemical reactions can takeplace in the cell.

A battery separator must be thin, so that its internal resistance is aslow as possible and high packing density can be achieved. This is theonly way to achieve good performance data and high capacitances. It isalso essential for the separators to soak up the electrolyte, and whenthe cells are full to ensure the exchange of gases. Whereas beforefabrics or the like were used, nowadays most separators are made frommicroporous materials such as fleeces and membranes.

In lithium batteries, the occurrence of short circuits is a problem.Under thermal load, the battery separator in lithium ion batteries isprone to melt, resulting in a short circuit with disastrousconsequences. Similar dangers exist if the lithium batteries are damagedmechanically or overcharged by chargers with faulty electronics.

In order to increase the safety of lithium ion batteries, shut-offmembranes were developed. These special separators close their poresvery rapidly at a given temperature, which is significantly lower thanthe melting point or ignition point of lithium. This largely preventsthe catastrophic effects of a short circuit in lithium batteries.

At the same time, however, high mechanical strength is also desirable inseparators, and this is lent to them by materials with high meltingtemperatures. For example, polypropylene membranes are advantageousbecause of their good resistance to perforation, but at about 164° C.the melting point of polypropylene is very close to the flame point oflithium (170° C.).

It is known in the related art to combine polypropylene membranes withother layers constructed from materials that have a lower melting point,for example polyethylene. Of course, such modifications of theseparators must not impair the other properties such as porosity, norhinder ion migration. However, the overall effect of includingpolyethylene layers on the permeability and mechanical strength of theseparator is very negative. It is also difficult to get the polyethylenelayers to adhere to polypropylene, and these layers can only be joinedby laminating, or only selected polymers of both classes can beco-extruded.

There are essentially four, different methods for manufacturing foilswith high porosities known in the related art: filler methods, coldstretching, extraction methods, and β-crystallite methods. These methodsdiffer fundamentally in the various mechanisms by which the pores arecreated.

For example, porous foils can be manufactured by adding very largequantities of filler materials. When they are stretched, the pores arecreated by the incompatibility between the filler materials and thepolymer matrix. In many applications, the large quantities of as much as40% by weight filler materials are associated with undesirable sideeffects. For example, the mechanical strength of such porous foils isreduced by the large content of filler materials despite stretching.Moreover, their pore size distribution is very wide, so that theseporous foils are essentially unsuitable for use in lithium ionbatteries.

In the “extraction methods”, the pores are created in principle byeluting a component from the polymer matrix with suitable solvent. Inthis context, a wide range of variants have been developed, and theydiffer in the types of additives and the suitable solvents that areused. Both organic and inorganic additives can be extracted. Thisextraction may be carried out as the last process step in themanufacture of the foil or it may be combined with a subsequentstretching step.

An older method that has proven successful in practice relies onstretching the polymer matrix at very low temperatures (coldstretching). For this, the foil is first extruded in the normal way andthen it is tempered for several hours to increase its crystallinecontent. In the following process step, it is cold stretched lengthwiseat very low temperatures to create a large number of faults in the formof tiny microcracks. This prestretched, intentionally flawed foil isthen stretched in the same direction again, with higher factors and atelevated temperatures, so that the flaws are enlarged to create poresthat form a network-like structure. These foils combine high porositieswith good mechanical strengths in the direction in which they arestretched, generally the lengthwise direction. However, their mechanicalstrength in the transverse direction remains unsatisfactory, which inturn means that their resistance to perforation is poor and they have ahigh tendency to splice in the lengthwise direction. The method is alsogenerally expensive.

Another known method for producing porous foils is based on the additionof β-nucleating agents to polypropylene. In the presence of theβ-nucleating agent, the polypropylene forms “β-crystallites” in highconcentrations as the melt cools down. In the subsequent lengthwisestretching, the β-phase is converted into the alpha modification of thepolypropylene. Since these different crystal forms vary in density,initially a large number of microscopic flaws are created here too, andthey too are expanded to create pores by the stretching. The foils thatare produced by this method have high porosities and good mechanicalstrengths both longitudinally and transversely and are extremelyinexpensive. These foils will be referred to as β-porous foils in thefollowing.

It is known that porous foils which are manufactured according to theextraction method may be provided with a shut-off function by theaddition of a low-melting component. Since in this method orientationtakes place first and the pores are created on the orientated foilafterwards by extraction, the low-melting component cannot hinder theformation of pores. Membranes with shut-off function are therefore oftenproduced by this method.

Low-melting components may also be added to lend a shut-off function inthe cold stretching method. The first stretching step must be carriedout at very low temperatures anyway, in order to create the microcracksin the first place. The second, orientation step is generally performedin the same direction, usually MD, and may therefore also take place ata relatively low temperature, since the molecule chains are notre-orientated. The mechanical properties of these foils are deficientparticularly in the transverse direction.

As an alternative, methods were developed in which various single-layerfoils with different functions are first produced separately, then theseare joined, that is to say laminated, to form a membrane with shut-offfunction. In this case, it is possible to optimise each layerindividually with respect to its desired function without running therisk that that porosity of the membrane might be impaired by theshut-off function. Of course, these methods are very expensive andtechnically involved.

Membranes consisting of β-porous foils have the drawback that until nowthey could only be provided with a corresponding shut-off function bylaminating in this way. In order to create adequate porosities togetherwith the desired mechanical strengths using β-crystallites andsubsequent biaxial stretching, the foil must first be orientatedlongitudinally and then stretched transversely. Transverse stretching ofa foil that has already been orientated longitudinally represents a defacto re-orientation of the polymer molecules and is contingent onsignificantly greater mobility of the polymer chains than is necessaryfor the first, lengthwise orientation of the unstretched polymers.Accordingly, transverse stretching of a polypropylene foil that hasalready been orientated longitudinally requires an elevated temperature,considerably higher than the desired shut-off temperature.

In the course of experiments relating to the present invention, it wastherefore expected that the pores created by lengthwise and transversestretching would be closed again by a low-melting component in theshut-off layer as early as the transverse stretching stage to such adegree that the porosity would be substantially limited. Lowering thetransverse stretching temperature is subject to mechanical limits, sincethe longitudinally stretched polypropylene can only be stretchedtransversely at temperatures of at least 145° C., and generallyundergoes transverse stretching at temperatures from 150 to 160° C.Consequently, there is no method known in the related art—except forlamination—by which β-porous foils can be provided with a shut-offfunction.

The object of the present invention consisted in providing a porousfoil, or a separator for batteries, which would have a shut-offfunction, high porosities and excellent mechanical strength. It shouldalso be possible to produce the membrane by simple, environmentallyresponsible and inexpensive methods.

The task underlying the invention is solved with a biaxially oriented,multilayer, microporous foil with shut-off function whose microporosityis created by converting β-crystalline polypropylene when the foil isstretched, and which comprises at least one shut-off layer I and atleast one porous layer II, wherein the shut-off layer includes propylenehomopolymer and propylene block copolymer and β-nucleating agent andpolyethylene, and the porous layer II includes propylene homopolymer andpropylene block copolymer and β-nucleating agent, and wherein the foilhas a Gurley value of 50 to 5000 s, an e-modulus in the lengthwisedirection of >300 N/mm², and in the transverse direction of >500 N/mm²,and after exposure to a temperature of 130° C. for 5 minutes the foilexhibits a Gurley value of at least 5000 s, wherein the Gurley valueafter this temperature treatment is at least 1000 s higher than before.

Surprisingly, the foil according to the invention exhibits highporosities, very good mechanical strength and the desired shut-offfunction. The Gurley value of the foil according to the invention isgenerally in a range from 50-5000 s; preferably 100 to 2000 s,particularly 120 to 800 s. The gas permeability of the foil issignificantly reduced if the foil is exposed to an elevated temperature.For the purposes of the present invention, this function is referred toas the “shut-off function”. Analysis is generally carried out inaccordance with the method described for analysing gas permeability,this measurement being taken before and after thermal loading of thefoil. For example, the Gurley value of the foil rises to at least 5000s, preferably to at least 8000 s, particularly to at least 10,000 to250,000 s after heat treatment at 130° C. lasting 5 minutes, wherein theGurley value with this heat treatment increases by at least 1000 s,preferably by 5000 to 250,000 s, and particularly by 10,000 to 200,000s. The Gurley value indicates (in secs) how it takes for a givenquantity of air (100 cm³) to diffuse through a defined area of the foil(1 inch²). The maximum value may thus be an infinite period of time.Accordingly, the second Gurley value, that is to say the Gurley valueafter heat treatment, which is used to describe a shut-off function, isa range with no upper limit. Ideally, the membrane is completelyimpermeable after the thermal treatment and does not allow any more airto pass at all, meaning that the Gurley value is infinite. The e-modulusof the foil according to the invention is 300 to 1800 N/mm2, preferably400 to 1500 N/mm2, and particularly 600 to 1200 N/mm2 in the lengthwisedirection, and 500 to 3000 N/mm2, preferably 800 to 2500 N/mm2, andparticularly 1000 to 2200 N/mm2 in the transverse direction.

When used as a separator in batteries as provided for in the presentinvention, the microporous foil is capable of effectively preventing theconsequences of a short circuit. If elevated temperatures occur insidethe battery due to a short circuit, the pores of the separator are closeby the shut-off layer rapidly in such manner as to prevent any furthergases or ions from passing through, thereby halting the chain reaction.

Surprisingly, the foil exhibits very high porosities despite theaddition of polyethylene in the shut-off layer. This is surprising fortwo reasons. The polyethylene content of, for example, 20% by weight inthe polymer mixture of the shut-off layer results in a smallerpercentage of β-crystallites in the cooled polymer mass of the shut-offlayer, and thus also to a lower β-crystallite content in the foil. Forpolypropylene foils without a polyethylene additive, the porosity isdetermined directly by the proportion of β-crystallites. The fewerβ-crystallites there are in the cooled, unstretched polypropylene foil,the lower the porosity that is formed after the PP foil is stretched.Surprisingly however, the porosity of the foil according to theinvention with polyethylene in the shut-off layer is no worse than apolypropylene foil having similar composition and being produced in thesame way but containing no polyethylene in the shut-off layer, eventhough the fraction of β-crystallite in the unstretched prefilm islower. It was also expected that because of layer II of the multilayerfoil, which consists entirely of propylene polymers, the transversestretching temperature would still have to be so high that thepolyethylene in the shut-off layer would cause the pores to close duringtransverse stretching because of its low melting point, which would alsocontribute to significant inhibition of good porosity. Surprisingly, itis possible to lower the transverse stretching temperature forstretching the polypropylene foil to the point that the polyethylenedoes not negatively affect the porosity, yet the foil, which alsocomprises a polypropylene layer without polyethylene, may still bestretched enough to achieve good mechanical strengths. At the same time,it was found that a quantity of polyethylene that is sufficient totrigger the shut-off effect does not at the same time ruin the porosity.Thus, surprisingly, it has been possible to provide a foil that exhibitshigh porosities, due to the biaxial stretching of β-crystallites, goodmechanical strengths, and a shut-off effect.

The foil according to the invention comprises at least one shut-offlayer I and at least one further porous layer II. All layers of the foilinclude a propylene homopolymer and propylene block copolymer,polyethylene, and at least one β-nucleating agent as the primarycomponents, and possibly small quantities of other polyolefins providingthey do not negatively affect the porosity and other importantproperties, and usual additives as required, for example stabilisers,neutralisers, each in effective quantities. It is essential for thepurposes of the invention that the shut-off layer also contains apolyethylene. The porous layer II preferably does not containpolyethylene nor any other polyolefin-type components that have amelting point <140° C., in particular this layer II is constructedalmost exclusively (>90% by weight) from polypropylene polymers. Thecomponents that are used in both the shut-off layer I and in the porouslayer II (propylene homopolymer and propylene block copolymer andβ-nucleating agent) will be described individually in the following.

Suitable propylene homopolymers contain 98 to 100% by weight, preferably99 to 100% by weight propylene units, and have a melting point (DSC) of150° C. or higher, preferably 155 to 170° C., and in general a melt-flowindex of 0.5 to 10 g/10 min, preferably 2 to 8 g/10 min, at 230° C. anda force of 2.16 kg (DIN 53735). Isotactic propylene homopolymers with ann-heptane-soluble fraction of less than 15% by weight, preferably 1 to10% by weight are preferred propylene homopolymers for the layer.Advantageously, isotactic propylene homopolymers with high chainisotacticity of at least 96%, preferably 97-99% (¹³C—NMR; triad method)may also be used. These raw materials are known in the related art asHIPP (High Isotactic Polypropylene) or HCPP (High CrystallinePolypropylene) polymers, and are characterized by the highstereoregularity of their polymer chains, higher crystallinity and ahigher melting point (compared with propylene polymers that have a¹³C—NMR isotacticity of 90 to <96%, which may also be used).

In addition, the shut-off layer I and the porous layer II each include apropylene block copolymer as a further component as well. Propyleneblock copolymers of such kind have a melting point above 140 and up to170° C., preferably from 150 to 165° C., particularly from 150 to 160°C., and a melting range that begins above 120° C., preferably in a rangefrom 125-140° C. The comonomer content, which is preferably ethylene, isfor example between 1 and 20% by weight, preferably between 1% and 10%by weight. The melt flow index of propylene block copolymers isgenerally in a range from 1 to 20 g/10min, preferably 1 to 10 g/10 min.

Both the shut-off layer I and the porous layer II may also include otherpolyolefins in addition to the propylene homopolymer and the propyleneblock copolymer, provided they do not negatively affect the properties,particularly the porosity, mechanical strengths and the shut-offfunction. Other polyolefins are for example statistical copolymers ofethylene and propylene with an ethylene content of 20% by weight orless, statistical copolymers of propylene with C₄-C₈ olefins having anolefin content of 20% by weight or less, terpolymers of propylene,ethylene and butylene having an ethylene content of 10% by weight orless and having a butylene content of 15% by weight or less, or otherpolyethylenes, such as LDPE, VLDPE and LLDPE.

In general, all known additives that promote the formation of β-crystalsof polypropylene when a polypropylene melt is cooled are suitable foruse as β-nucleating agents for both layers I and II. Such β-nucleatingagents, and their mode of action in a polypropylene matrix, are known intheir own right from the prior art, and will be described in detail inthe following.

Various crystalline phases of polypropylene are known. When a moltenmass cools, it is usually mainly α-crystalline PP that forms, with amelting point at approximately 158-162° C. By implementing a certaintemperature program, it is possible to ensure that a small proportion ofa β-crystalline phase is formed upon cooling, with a melting point in arange from 140-150° C., markedly lower than that of the monoclinicα-modification. Additives are known in the related art that causeformation of a higher proportion of the β-modification whenpolypropylene cools, including for example γ-quinacridone,dihydroquinacridine, or calcium salts of phthalic acid.

For the purposes of the present invention, preferably highly activeβ-nucleating agents are used, which form a β-fraction of 40-95%,preferably 50-85% (DSC), when a propylene homopolymer melt (PP-fraction100%) cools down. The β-fraction is determined from the DSC of thecooled propylene homopolymer melt. For example a two-componentβ-nucleating system of calcium carbonate and organic dicarboxylic acidsas preferred, such as is described in DE 3610644, which document isexpressly included herewith by reference. Particularly advantageous arecalcium salts of dicarboxylic acids such as calcium pimelate or calciumsuberate, as are described in DE 4420989, which is also expresslyincluded by reference. The dicarboxamides described in EP-0557721,particularly N,N-dicyclohexyl-2,6-naphthalene dicarboxamide, are alsosuitable β-nucleating agents.

Besides the β-nucleating agents, in order to obtain a high fraction ofβ-crystalline polypropylene it is also important to maintain a certaintemperature range and residence times at these temperatures as the meltfilm is cooling. Cooling of the melt film preferably takes place at atemperature of 60 to 140° C., particularly 80 to 130° C. A slow coolingprocess also promotes the growth of the β-crystallites, so the drawingspeed, that is to say the speed at which the melt film runs over thefirst, cooling roller, should be slow to ensure that the necessaryresidence times at the selected temperatures are long enough. Thedrawing speed is preferably less than 25 m/min., particularly 1 to 20m/min.

Particularly preferred embodiments of microporous foil according to theinvention contain 50 to 10,000 ppm, preferably 50 to 5000 ppm,particularly 50 to 2000 ppm calcium pimelate or calcium suberate as theβ-nucleating agent in each layer.

The shut-off layer I generally contains 45 to 75% by weight, preferably50 to 70% by weight propylene homopolymer and 10-45% by weight,preferably 20 to 35% by weight propylene block copolymer, and 15 to 45%by weight, preferably 15 to 30% by weight polyethylene, and 0.001 to 5%by weight, preferably 50-10,000 ppm of at least one β-nucleating agentrelative to the weight of the shut-off layer. If additional polyolefinsare included in the shut-off layer, the proportion of the propylenehomopolymer or the block copolymer is reduced correspondingly. Ingeneral, if they are also included, the quantity of additional polymersin the shut-off layer will be from 0 to <10% by weight, preferably from0 to 5% by weight, particularly from 0.5 to 2% by weight. Similarly, theproportion of propylene homopolymer or propylene block copolymer will bereduced as above if larger quantities of up to 5% nucleating agent areused. Additionally, the shut-off layer may also contain usualstabilisers and neutralising agents, and if required other additives inthe usual low quantities of less than 2% by weight.

For the purposes of this invention, preferred polyethylenes in theshut-off layer are HDPE or MDPE. Generally, like HDPE and MDPE, thesepolyethylenes are not compatible with polypropylene and form a separatephase in the mixture with polypropylene. The presence of a separatephase is demonstrated for example in a DSC measurement by a separatemelting peak in the range of the melting temperature of polyethylene,generally in a range from 115-145° C. HDPE generally has an MFI (50N/190° C.) greater than 0.1 to 50 g/10 min, preferably 0.6 to 20 g/10min, measured in accordance with DIN 53 735 and a crystallinity of 35 to80%, preferably 50 to 80%. The density, measured at 23° C. in accordancewith DIN 53 479, method A, or ISO 1183, is in the range from >0.94 to0.97 g/cm³. The melting point, measured with DSC (maximum of the meltingcurve, heating rate 20° C./min), is between 120 and 145° C., preferably125-140° C. Suitable MDPE generally has an MFI (50 N/190° C.) of greaterthan 0.1 to 50 g/10 min, preferably 0.6 to 20 g/10 min, measured inaccordance with DIN 53 735. The density, measured at 23° C. inaccordance with DIN 53 479, method A, or ISO 1183, is in the rangefrom >0.925 to 0.94 g/cm³. The melting point, measured with DSC (maximumof the melting curve, heating rate 20° C./min), is between 115 and 130°C., preferably 120-125° C.

It is also advantageous for the purposes of the invention if thepolyethylene has a narrow melting range. This means that in a DSC of thepolyethylene the start of the melting range and the end of the meltingrange are no more than 10K, preferably 3 to 8K apart. For thesepurposes, the extrapolated onset is taken as the start of the meltingrange, and the extrapolated end of the melting curve is correspondinglytaken to represent the end of the melting range (heating rate 10K/min).

The parameters “melting point” and “melting range” are determined by DSCmeasurement and read off from the DSC curve, as described for themeasuring methods.

The porous layer II generally includes 50 to 85% by weight, preferably60 to 75% by weight propylene homopolymer, and 15 to 50% by weight,preferably 25 to 40% by weight propylene block copolymer, and 0.001 to5% by weight, preferably 50-10,000 ppm of at least one β-nucleatingagent relative to the weight of the layer as well as any additives suchas the stabilisers and neutralising agents referred to above.

If additional polyolefins are included in the porous layer II, theproportion of the propylene homopolymer or the block copolymer isreduced correspondingly. In general, if they are also included, thequantity of additional polymers will be from 0 to <20% by weight,preferably from 0.5 to 15% by weight, particularly from 1 to 10% byweight. Similarly, the proportion of propylene homopolymer or propyleneblock copolymer will be reduced as above if larger quantities of up to5% nucleating agent are used. Additionally, the porous layer II may alsocontain usual stabilisers and neutralising agents, and if required otheradditives in the usual low quantities of less than 2% by weight. Ingeneral, the porous layer II does not contain any additional HDPE and/orMDPE, so that the mechanical strength of this layer II may be optimised.But the same applies for these HDPEs and MDPEs as for the otheradditional polymers, that is to say small quantities that do not impairthe foil properties, particularly the porosity, the shut-off functionand the mechanical properties, are possible. The quantity of HDPEs andMDPEs in the porous layer II is preferably less than 5% by weight,particularly in the range of 0-1% by weight.

The composition according to the invention of the foil comprisingpropylene homopolymer, propylene block copolymer, β-nucleating agent andpolyethylene exhibits a characteristic pattern of at least 3 peaks inthe DSC measurement during the second melting process. These peaks areattributable to the α-crystalline phase of the propylene homopolymer,the β-crystalline phase of the propylene homopolymer, and thepolyethylene. According to a DSC measurement, the foil according to theinvention thus has one peak in the range from 115-145° C. for thepolyethylene, one peak in the range from 140-155° C. for theβ-crystalline polypropylene, and a third peak in the range from 155-175°C. for the α-crystalline polypropylene.

The microporous membrane foil consists of multiple layers. The thicknessof the membrane foil is generally in a range from 10 to 100 μm,preferably 15 to 80 μm. The microporous foil may be subject to a corona,flame or plasma treatment to improve its filling with electrolyte. Thethickness of the porous layer II is in a range from 9 μm to 60 μm,preferably 15 to 50 μm. The thickness of the shut-off layer I is in arange from 1 μm to 40 μm, preferably 3 to 30 μm.

The microporous foil may include further porous layers that areconstructed similarly to the porous layer II, wherein the composition ofthese further layers does not necessarily have to be identical to thatof the porous layer II, but may be the same. Foils with three layerspreferably have an inner shut-off layer I that is covered on both sidesby porous layers II.

The density of the microporous foil is generally in a range from 0.1 to0.6 g/cm³, preferably 0.2 to 0.5 g/cm³. In order to be used as theseparator in batteries, the foil should have a Gurley value from 50 to5000 s, preferably from 100 to 2500 s. The bubble point of the foilshould not be above 350 nm, it should preferably be in the rang from 50to 300 nm, and the average pore diameter should be in the range from 50to 100 nm, preferably in the range from 60-80 nm.

The porous foil according to the invention is preferably produced in theknown flat film extrusion process. During this process, the mixtures ofpropylene homopolymer, propylene block copolymer, β-nucleating agent inthe respective layer and polyethylene for the shut-off layer are mixed,melted in an extruder and co-extruded together and simultaneouslythrough a flat nozzle onto a take-off roller, on which the multilayermelt film solidifies and cools, forming the β-crystallites. The coolingtemperatures and times are programmed such that the highest possiblefraction of β-crystalline polypropylene is formed in the prefilm. Thecontent of β-crystals is slightly lower than in pure polypropylenefoils, because of the polyethylene fraction in the shut-off layer. Ingeneral, the content of β-crystallites in the prefilm is 30-85%,preferably 50-80%, particularly 60-70%. This prefilm with high contentof β-crystalline polypropylene is then stretched biaxially in such a waythat the β-crystallites are converted into α-polypropylene during thestretching, and a lattice-like porous structure is formed. Finally, thebiaxially stretched foil undergoes heat setting, and possibly surfacecorona, plasma or flame treatment.

The biaxial stretching (orientation) is generally carried out inconsecutive steps, and the material is preferably stretched lengthwisefirst (in the direction of the machine) and then transversely(perpendicularly to the machine).

The take-off roller or rollers are kept at a temperature of 60 to 135°C., preferably 100 to 130° C., to promote formation of a high fractionof β-crystalline polypropylene in both layers.

When stretching lengthwise, the temperature is below 140° C., preferably70 to 120° C. The longitudinal stretching ratio is in the range from 2:1to 5:1, preferably from 3:1 to 4.5:1. Transverse stretching takes placeat a temperature from 120-145° C., which should be selected such thatthe transverse stretching temperature is not substantially higher thanthe melting point of the polyethylene. In general, the transversestretching temperature may be up to 5° C., preferably up to 3° C. abovethe melting point of the polyethylene. If the transverse stretchingtemperature is below the melting point of the polyethylene, thedifferences may be greater, for example up to 20° C., preferably up to10° C. In this case, the transverse stretching temperature will beprogrammed on the basis of the stretchability of the polypropylenecontent in the foil. The transverse stretching ratio is in a range from2:1 to 9:1, preferably 3:1-8:1.

Lengthwise stretching may be performed expediently using two rollersrunning at different speeds corresponding to the desired stretchingratio, and transverse stretching with an appropriate tenter.

The biaxial foil stretching process is generally followed by thermalfixing (heat treatment), wherein the foil is exposed to a temperature of110 to 140° C. for about 0.5 to 500 s, preferably 10 to 300 s, forexample via rollers or an air heater box. The temperature in thermalfixing should be set such that the temperature the foil reaches as itpasses through the fixing field is lower than the melting point of thepolyethylene, or not more than 1 to 2° C. above it. The foil is thenrolled up in the normal way with a takeup mechanism.

As indicated above, if applicable one surface of the foil may besubjected to one of the known corona, plasma or flame treatment methodsafter biaxial stretching.

The following measuring methods were used to characterize the rawmaterials and foils:

Melt Flow Index

The melt flow index of the propylene polymers was measured in accordancewith DIN 53 735 under a load of 2.16 kg and at 230° C., and at 190° C.with a load of 2.16 kg for polyethylenes.

Melting Points and Melting Ranges

Because of their different crystalline ranges, or phases, partlycrystalline thermoplastic polymers such as propylene polymers do nothave a single defined melting point, but rather a melting range. Meltingpoint and melting range are therefore values that are derived veryaccurately from a DCS curve for the respective polymer. In DSCmeasurement, a quantity of heat per unit of time is introduced to thepolymer at a defined heating rate, and the heat flux is plotted againstthe temperature, that is to say the change in enthalpy is measured asthe divergent course of the heat flux from the baseline. The baseline isunderstood to be the (linear) component of the curve in which no phaseconversions are taking place. Here, the heat quantity applied and thetemperature are in a linear relationship with one another. In the rangein which melting processes take place, the heat flux increases by theenergy required for melting and the DSC curve climbs. In the range inwhich most crystallites are melting, the curve reaches a maximum valueand falls back to the baseline. For the purposes of the presentinvention, the melting point is the maximum value of the DSC curve. Forthe purposes of the present invention, the start of the melting range isthe temperature at which the DSC curve deviates from the baseline andthe DSC curve begins to rise. Conversely, the end of the melting rangeis the temperature at which the DSC curve has fallen back to the levelof the baseline. The temperature difference between the start and end isthe melting range.

In order to determine the melting point and the melting range, thesample is melted and cooled again for the first time in a range from 20to 200° C. and with a heating and cooling speed of 10K/1 min. Then, asecond DSC curve is recorded (20-200° C. and 10K/1 min) in the normalway and under the same conditions, and this second heating curve isevaluated as described.

β-Content of the Prefilm

The β-content of the prefilm is also determined by DSC measurement,which is carried out on the prefilm as follows: first, the prefilm isheated to 220° C. and melted in the DSC at a heating rate of 10K/min,then cooled again. From the first heating curve, the degree ofcrystallinity K_(β,DSC) is determined as a ratio of the enthalpies offusion of the β-crystalline phase (H_(β)) to the total of the enthalpiesof fusion for the β- and α-crystalline phases (H_(β)+H_(α)).

Density

Density is determined in accordance with DIN 53 479, method A.

Permeability (Gurley Value)

The permeability of the foils was measured in accordance with ASTM D726-58 using the 4110 Gurley Tester. The time taken by 100 cm³ air topermeate through the label area of 1 inch² (6.452 cm²) is determined inseconds. The pressure differential over the foil corresponds to thepressure of a 12.4 cm high water column. The time taken is then recordedas the Gurley value.

Shut-Off Function

The shut-off function is determined by Gurley measurements before andafter a heat treatment at a temperature of 130° C. The Gurley value ofthe foil is measured as described in the preceding. Then, the foil isexposed to a temperature of 130° C. for five minutes in a heatingfurnace. After this, the Gurley value is calculated again as described.The shut-off function is considered to be in effect when the foil has aGurley value of at least 5000 s and has increased by at least 1000 safter the heat treatment.

The invention will now be explained with the following examples.

EXAMPLE 1

After the co-extrusion process, a double-layer prefilm (porous layer IIand shut-off layer I) was co-extruded through a flat sheet die at anextrusion temperature of 240 to 250° C. This prefilm was first drawn offand cooled on a cooling roller. Then, the prefilm was orientatedlongitudinally and transversely, and finally heat-set. The compositionof the foil was as follows:

Shut-Off Layer I:

Approx. 60% by weight high isotactic propylene homopolymerisate (PP)with a ¹³C—NMR isotacticity of 97% and an n-heptane soluble fraction of2.5% by weight (relative to 100% PP) and a melting point of 165° C.; anda melt flow index of 2.5 g/10 min at 230° C. under a load of 2.16 kg(DIN 53 735), andApprox. 20% by weight HDPE (High Density Polyethylene) having a densityof 0.954 (ISO 1183) and an MFI of 0.4 g/10 min at 190° C. under a loadof 2.16 kg (ISO 1133/D) or 27 g/10 min at 190° C. under a load of 21.6kg (ISO 1333/G), and melting point of 130° C. (DSC: peak at 10K/minheating rate), the melting range begins at 126° C. and ends at 133° C.Approx. 20% by weight Propylene-ethylene block copolymerisate used withan ethylene fraction of 5% by weight relative to the block copolymer andan MFI (230° C. and 2.16 kg) of 6 g/10 min and a melting point (DSC) of165° C., and 0.04% by weight Ca pimelate as the β-nucleating agent.

Porous Layer II:

Approx. 80% by weight high isotactic propylene homopolymerisate (PP)with a ¹³C—NMR isotacticity of 97% and an n-heptane soluble fraction of2.5% by weight (relative to 100% PP) and a melting point of 165° C.; anda melt flow index of 2.5 g/10 min at 230° C. under a load of 2.16 kg(DIN 53 735), andApprox. 20% by weight Propylene-ethylene block copolymerisate used withan ethylene fraction of 5% by weight relative to the block copolymer andan MFI (230° C. and 2.16 kg) of 6 g/10 min and a melting point (DSC) of165° C., and 0.04% by weight Ca pimelate as the β-nucleating agent

The foil also contained the standard, small quantities of stabiliser andneutralising agent in both layers.

In detail, the following conditions and temperatures were selected forproducing the foil:

-   Extrusion: Extrusion temperature 235° C.-   Take-off roller: Temperature 125° C.,-   Drawing speed: 4 m/min-   Longitudinal stretching: Stretching roller T=90° C.-   Longitudinal stretching by Factor 3.0-   Transverse stretching: Heating panels T=125° C.-   Draw panels T=125° C.-   Transverse stretching by Factor 5.0-   Heat-setting: T=125° C.

The porous foil thus created was about 25 μm thick, the shut-off layermaking up 12 μm of the total thickness. The foil had a density of 0.38g/cm³, and had an even, white-opaque appearance.

EXAMPLE 2

A two-layer foil was produced as described in example 1. In contrast toexample 1, the fraction of propylene homopolymer in the shut-off layerwas reduced to 55% by weight and the fraction of HDPE was increased to25% by weight. The composition of the porous layer II and the processconditions were unchanged. The porous foil produced in this way wasabout 28 μm thick, each layer having a thickness of 14 μm. The film hada density of 0.42 g/cm³ and had an even, white-opaque appearance.

EXAMPLE 3

A two-layer foil was produced as described in example 1. In contrast toexample 1, the fraction of propylene homopolymer in the shut-off layerwas reduced to 40% by weight and the fraction of HDPE was increased to40% by weight. The rest of the composition of the porous layer II andthe process conditions were unchanged. The foil produced in this way was30 μm thick, each layer having a thickness of 15 μm. The film had adensity of 0.42 g/cm³ and had an even, white-opaque appearance.

EXAMPLE 4

A two-layer foil was produced as described in example 1. In contrast toexample 1, the HDPE in the shut-off layer was replaced with an MDPEhaving a density of 0.954 g/cm³ (ISO 1183) and an MFI of 0.4 g/10 min at190° C. under 2.16 kg load (ISO 1133/D) or 27 g/10 min at 190° C. under21.6 kg load (ISO 1333/G) and a melting point of 125° C. (DSC: peak at10° C./min heating rate). The melting range of the MDPE is between120-127° C. The transverse stretching temperature was also lowered incomparison to example 1, to 120° C. The rest of the composition of theporous layer II and all other process conditions were unchanged. Thestretched foil was 30 μm thick, each layer having a thickness of about15 μm. The foil had a density of 0.42 g/cm³ and had an even,white-opaque appearance.

COMPARISON EXAMPLE 1

A foil was produced as described in example 1. In contrast to example 1,the foil did not comprise a shut-off layer and only consisted of porouslayer II, which was correspondingly thicker. The foil was thus producedas a single layer foil. The composition of the porous layer II and theprocess conditions were unchanged. The foil had a white-opaqueappearance, a thickness of 25 μm and a density of 0.38 g/cm³.

COMPARISON EXAMPLE 2

A foil was produced as described in example 1. The composition of thefoil was not changed. In contrast to example 1, in this case the foilwas now stretched transversely at a temperature of 135° C. The porousfoil produced in this way was about 25 μm thick and had a density of0.38 g/cm³ and had a white-opaque appearance.

TABLE Gurley Gurley value after thermal E-modulus in Example value [s]treatment (5 min @ 130° C.) [s] MD/TD [N/mm²] Ex. 1 350  9,000 1020/2100Ex. 2 350  10,000 1000/1850 Ex. 3 600 100,000  900/1400 Ex. 4 800 45,000 1010/2050 Comp. ex 1 400    600 1080/1260 Comp. ex 2 4600 18,500 1120/2120

1.-19. (canceled)
 20. A biaxially oriented, multilayer, microporous foilwith shut-off function whose microporosity is created by convertingβ-crystalline polypropylene when the foil is stretched, and whichcomprises at least one shut-off layer I and at least one porous layerII, wherein the shut-off layer contains propylene homopolymer andpropylene block copolymer and β-nucleating agent and polyethylene, andthe porous layer II contains propylene homopolymer and propylene blockcopolymer and β-nucleating agent, wherein the foil has a Gurley value of50 to 5000 s, an e-modulus in the lengthwise direction >300 N/mm²,and >500 N/mm² in the transverse direction, and after exposure to atemperature of 130° C. for 5 minutes the foil exhibits a Gurley value ofat least 5000 s, wherein the Gurley value after this temperaturetreatment is at least 1000 s higher than before.
 21. The foil as claimedin claim 20, wherein the polyethylene of the shut-off layer I has amelting point of 115-140° C.
 22. The foil as claimed in claim 20,wherein the melting range of the polyethylene of shut-off layer I has amaximum width of 10 K.
 23. The foil as claimed in claim 20, wherein thepolyethylene in the shut-off layer I is an HDPE or an MDPE.
 24. The foilas claimed in claim 20, wherein the shut-off layer I contains 15-45% byweight polyethylene relative to the weight of the shut-off layer. 25.The foil as claimed in claim 20, wherein the shut-off layer I contains45 to 75% by weight propylene homopolymer, 10 to 45% by weight propyleneblock copolymer and 50 to 10,000 ppm β-nucleating agent.
 26. The foil asclaimed in claim 20, wherein the propylene homopolymer is a highisotactic polypropylene having a chain isotaxy (¹³C—NMR) of 96 to 99%.27. The foil as claimed in claim 20, wherein the propylene homopolymeris an isotactic polypropylene having a chain isotaxy (¹³C—NMR) of 90 to<96%.
 28. The foil as claimed in claim 20, wherein the nucleating agentis a calcium salt of pimelic acid or of suberic acid or is acarboxamide.
 29. The foil as claimed in claim 20, wherein the porouslayer II contains 50 to 85% by weight propylene homopolymer, 15 to 50%by weight propylene block copolymer, and 50 to 10,000 ppm β-nucleatingagent.
 30. The foil as claimed in claim 20, wherein the porous layer IIcontains 0 to 5% by weight HDPE and/or MDPE.
 31. The foil as claimed inclaim 20, wherein the foil comprises a further porous layer thatcontains propylene homopolymer and propylene block copolymer andβ-nucleating agent.
 32. The foil as claimed in claim 20, wherein thedensity of the foil is in a range from 0.1 to 0.6 g/cm³.
 33. The foil asclaimed in claim 20, wherein the foil has a Gurley value of 50 to 5000 sand has a Gurley value of at least 8000 s after it has been exposed to atemperature of 130° C. for five minutes.
 34. The foil as claimed inclaim 20, wherein the foil has a thickness of 10 to 100 μm.
 35. A methodfor producing the foil as claimed in claim 20, which comprises producingthe foil according to the flat film process and the take-off rollertemperature is in a range from 60 to 130° C.
 36. The method as recitedin claim 35, wherein the unstretched prefilm has a β-crystallite contentof 30 to 85%.
 37. The method as recited in claim 35, wherein the foil isstretched transversely at a temperature no more than 2° C. higher thanthe melting point of the polyethylene.
 38. A separator in a batterywhich comprises the foil as claimed in claim
 20. 39. A separator in arechargeable battery which comprises the foil as claimed in claim 20.